1. Field of the Invention
This invention relates to new mixtures of certain polyurethane prepolymers containing tertiary amino nitrogen atoms and aromatically-bound isocyanate groups with certain auxiliaries and additives, which mixtures, are on the one hand, effectively storable in the absence of moisture and, on the other hand, harden extremely quickly under the influence of atmospheric moisture. The present invention also relates use of these prepolymers as adhesives or coatings for various flexible or rigid substrates.
2. Description of the Prior Art
Polyurethane prepolymers having aromatically-bound isocyanate groups and incorporated tertiary amino nitrogen atoms are very labile systems when viewed chemically because they are extremely reactive due to the incorporated catalyst. On the other hand, such systems are of considerable practical interest as a one-component system, for example as an adhesive or coating due to the simple processibility thereof. However, the one-component systems known hitherto, based on NCO prepolymers having incorporated tertiary amino nitrogen atoms have not yet been successful, particularly in the adhesive sector, because they have not yet been able to satisfy adequately the practical expectations with respect to physiological safety, good storability and short gel times.
Thus, solutions of aromatic polyisocyanates, inter alia, of NCO prepolymers containing tertiary nitrogen atoms, in certain solvents containing amide or sulphoxide groups, for example dimethylformamide or dimethyl sulphoxide, are known, for example, from U.S. Pat. No. 3,317,481. The nature of the solvents mentioned mitigates against use as an easy-to-handle adhesive since these solvents are pronounced high-boilers which remain for a long time in the adhesive film and are physiologically dangerous. Moreover, since they are aggressive solvents to many types of plastics, their use as an adhesive for plastics may cause a lasting swelling and deformation of the substrates to be bonded.
British Pat. No. 1,126,094 describes NCO prepolymers based on 4,4'-diisocyanato-diphenyl-methane, trifunctional, basic polyethers and conventional plasticizers. Adhesives based on such NCO prepolymers generally only have a very slight substrate adhesion, as shown by our own experiments. They are unsuitable for bonding conventional plastics, for example PVC of glass-reinforced polyester resin, because the adhesion of the adhesive film is not given on polyester-resin plastics mentioned.
German Auslegeschrift No. 1,922,626 describes NCO prepolymers based on basic polyethers and 2,4-diisocyanato-toluene. These NCO prepolymers are also unsuitable particularly as an adhesive for plastics for the above-mentioned reasons. Moreover, the use of diisocyanato-toluene requires additional measures due to the relatively high vapor pressure thereof, in order to reduce the content of this diisocyanate in the final product.
The leather lacquers based on NCO prepolymers described in published European Patent Application No. 15465 are equally unsuitable as a one-component adhesive, because the gel rate thereof is much too slow even in the presence of atmospheric moisture.
An object of the present invention is to provide new one-component systems which meet the above-mentioned practical requirements with respect to physiological safety, storage stability and gel time. This object may be achieved using the mixtures according to the present invention which is described in more detail below.